Conventionally, a step for manufacturing a sealed battery includes a leak testing step for checking airtightness of a battery case for the purpose of, for example, preventing battery performance from deteriorating due to infiltration of water into the battery case (see Patent Literature 1, for example).
A sealed battery used for an airtightness inspecting method disclosed in Patent Literature 1 is manufactured in an airtight chamber having an atmosphere of detection gas, and thereby the detection gas is introduced into a battery case. After the detection gas is introduced into the battery case, the battery case is sealed. The airtightness inspecting method disclosed in Patent Literature 1 includes producing a vacuum in the airtight chamber after removing the detection gas from the airtight chamber, detecting the detection gas leaked from the sealed battery by a gas sensor arranged in the vicinity of a vent of the airtight chamber.
In the above-mentioned the airtightness inspecting method disclosed in Patent Literature 1, since the sealed battery is manufactured under the atmosphere of the detection gas, equipment for manufacturing the battery needs to be sealed by a large airtight chamber. Moreover, a large amount of detection gas is required in order to produce the atmosphere of the detection gas in the airtight chamber.
There is the following method: under an air atmosphere, detection gas is jetted to be introduced into a battery case through a nozzle from the outside of the battery case before sealed, and then the battery case is conveyed to predetermined equipment for sealing the battery case.
However, if helium gas as the detection gas is introduced into the battery case for the purpose of, for example, preventing battery performance from being adversely affected, most of the helium gas with small specific gravity is leaked until sealing the battery case.
Therefore, in this case, rate of utilization of the detection gas (proportion of the amount of the stored detection gas to the amount of the jetted detection gas) is reduced.
Moreover, in this case, time from introduction of the helium gas until seal of the battery case varies, and thereby leakage of the helium gas varies. In other words, in this case, density of the helium gas in the battery case varies.
As shown in FIG. 23, if a predetermined amount of leak gas is leaked from the battery case, an output value of a gas sensor varies according to the density of the helium gas in the battery case. Concretely, the output value becomes large when the density of the helium gas in the leak gas is high (see graph G11 in FIG. 23), and becomes small when the density of the helium gas in the leak gas is low (see graph G12 in FIG. 23).
In a leak testing step, a threshold M1 needs to be set on the basis of the output value of the gas sensor when the density of the helium gas is low.
Accordingly, when the leak testing step is performed with respect to the battery case accommodating the helium gas with high density, the output value of the gas sensor may exceed the threshold M1 in spite of leakage of the leak gas smaller than leakage N of the leak gas corresponding to the threshold M1 when the density of the helium gas is low (see range R1 in FIG. 23).
Thus, if the detection gas is introduced under an air atmosphere, an amount of the helium gas present in the leak gas leaked from the battery case during the inspection varies, and thereby a sealed battery which normally passes the inspection fails the inspection with relatively high probability. In other words, if the detection gas is introduced under an air atmosphere, rate of erroneous determination may increase.
As mentioned above, since a conventional technique cannot minimize the leakage of the detection gas after the introduction of the detection gas into the battery case, the rate of the utilization of the detection gas may decreases and rate of erroneous determination in the inspection may increase.